The optical recording and readout of data has been described by numerous authors. Chapter 6 in the book "Advanced Digital Audio", Edited by Ken C. Pohlmann, SAMS, 1991; gives a comprehensive account of the present state of optical disk technology. Such recording systems are classified into Read Only Media (ROM) which can only be read out optically but have to use a different recording method; Write Once Read Many (WORM) media which can be written optically but only once, and Erasable Write Many Media which can be repeatedly written and read.
Optical recording and retrieval systems are limited in the density of data that can be recorded and read by the natural laws of lightwave diffraction. Such laws make it impossible to focus a light beam to a spot smaller in diameter than the wavelength of the light. Present systems are also limited in the speed of readout by the amount of light power or brightness that can be focussed to a very small diameter spot.
This has led to efforts to develop shorter wavelength high brightness light sources or Lasers. However such lasers emitting blue or shorter wavelengths have proved difficult to make and remain short lived, unreliable and expensive to produce. Other efforts have sought to defeat the effects of diffraction by keeping the recording medium very close to the output port of sub-wavelength sized fiber waveguides. Such waveguides are made by tapering glass fibers from hundred micron diameters down to a fine point of sub micron size diameter. Light launched into the large diameter side travels to the fine tip where it exits towards the recording medium. The drawback of such systems is the very large loss of light power caused by the tapered guides which causes very low light levels on the medium and severely limits the readout speed and write capability.
Another approach to increasing data densities utilizes holographic recording in three dimensional media. In such systems the interference pattern between a data carrying beam and a reference light beam is recorded in a light sensitive medium. By varying the angle between these beams independent records can be superimposed in a medium and selectively read out as described by F. H. Mok in Optics Letters, vol. 18, p.915 (1993). The disadvantages of such methods have been the difficulty of developing media sensitive in the wavelength range of available lasers; the complexity of optical readout systems with angular resolution; the instability of the records over time and temperature and the incompatibility of these systems with the presently utilized means of tracking and focussing.
Furthermore traditional optical data recording systems consist of a storage medium usually in disk form, with binary digital data recorded thereupon in the form of two physically distinct states. Thus since only two distinct states are used per recording spot or pixel, only one bit of information can be recorded per pixel. A variety of techniques are used to affect a change in the intensity or polarization of the light reflected from the recording surface. The techniques utilizing intensity modulation include the creation of small and large pits, the modulation of the distance between pits and the creation of bumps on a generally flat surface. Light from a laser source is focussed onto the data recording areas and then reflected back to a photodetector which transforms the power received into a proportional electrical current. While the disk is rotating, ones and zeroes are read out as high and low current levels in the photodetector.
The techniques utilizing polarization modulation include causing a change in the crystalline phase of the recording medium material, or a change in the state of magnetization. A polarized light beam is then focussed onto the data recording areas and suffers a change in its state of polarization upon reflection. The change in polarization is sensed by a detector-polarizer combination.
Because of noise and drift in the optical power from the lasers and because of possible asperities and dust particles in the beam path as well as the noise generated in the associated electronics; the modulation of intensities or polarizations used must be kept large. This has traditionally limited the modulation states to only two: high and low,corresponding to binary ones and zeroes and limiting storage to one bit per spot. Accordingly storage densities on all optically read disks are limited presently to 10.sup.8 bits/cm.sup.2 by light diffraction. These optical records are used in computer systems for data storage, entertainment systems for audio, video and program storage and in general data archiving systems.
One notes that prior art systems generally fail to concentrate a large enough optical power onto a small enough surface area to achieve high density recording without reducing the signal to noise ratio of the readout, and consequently impairing the bandwidth of the readout.